1. Field of the Invention
The present invention relates to a comparator using an operational amplifier, more particularly, it relates to a comparator circuit enabling switching between one state wherein a comparison is effected and another state wherein an input offset voltage is canceled, thus making it possible to obtain a precise comparison output voltage not affected by an input offset voltage of the operational amplifier.
2. Description of the Related Art
In the comparator using the operational amplifier, it is well known that decision level errors in a reference voltage used for comparison with the input signal are caused by the input offset voltage. The input offset voltage is that occurring when input terminals of the comparator are shorted together and a small voltage remains in an internal circuit of the comparator. Such an input offset voltage is caused by a dispersion of electrical characteristics of the circuit elements, for example, FET transistors, resistors and the like, in the comparator and by a temperature drift of the above circuit elements. When decision level errors are caused by the input offset voltage, a precise comparison output voltage can not be obtained from the comparator.
Conventionally, there have been attempts to reduce or cancel the dispersion of values of the circuit elements, as countermeasures against the input offset voltage. Such countermeasures, however, are not satisfactory because it is difficult to mass-produce precise circuit elements without dispersion of the electrical characteristic. Moreover, it is necessary to provide an external clock generator for generating a clock signal to control an offset voltage cancellation circuit. In this case, it is also necessary to change the frequency of the clock signal when the frequency of an input signal is changed, in order to carry out the offset voltage cancellation over a constant period.